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Class 11 Chemistry Chapter 9: Hydrocarbons MCQs with answers

10 multiple-choice questions on Hydrocarbons, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.

Question 1
Which of the following molecular formulae can belong to an alkane?
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Answer: (b) C₈H₁₈

Alkanes have the general formula CₙH₂ₙ₊₂; for n = 8 this is C₈H₁₈. C₇H₁₄ and C₉H₁₈ fit CₙH₂ₙ and C₆H₁₀ fits CₙH₂ₙ₋₂.

Question 2
Which of the following alkanes contains a quaternary (4°) carbon atom?
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Answer: (d) 2,2-dimethylbutane

In 2,2-dimethylbutane, CH₃–C(CH₃)₂–CH₂–CH₃, carbon 2 is bonded to four carbon atoms, so it is quaternary. The other three have at most tertiary carbons (bonded to three carbons).

Question 3
Which alkyl halide, treated with sodium in dry ether, gives 2,3-dimethylbutane as the only alkane?
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Answer: (b) 2-chloropropane

In the Wurtz reaction two alkyl groups of the halide join: two (CH₃)₂CH– groups give (CH₃)₂CH–CH(CH₃)₂, 2,3-dimethylbutane. 1-Chloropropane gives hexane; 1-chlorobutane gives octane; 2-chlorobutane gives 3,4-dimethylhexane.

Question 4
The sodium salt of a carboxylic acid gives ethane when heated with soda lime. Electrolysis of an aqueous solution of the same salt by Kolbe's method gives at the anode
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Answer: (b) butane

Decarboxylation removes one carbon, so the salt is sodium propanoate, CH₃CH₂COO⁻Na⁺. In Kolbe's method two ethyl radicals (formed after loss of CO₂) join: CH₃CH₂–CH₂CH₃, butane.

Question 5
Methane cannot be prepared by Kolbe's electrolytic method because
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Answer: (c) the alkane is formed by joining two alkyl free radicals, so it has at least two carbon atoms

At the anode carboxylate ions lose electrons, give alkyl free radicals and CO₂, and two radicals combine. Even the smallest radical, ĊH₃, gives CH₃–CH₃; methane is not formed.

Question 6
Which of the following alkanes is a liquid at 298 K?
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Answer: (d) C₁₂H₂₆

At 298 K, C₁ to C₄ alkanes are gases, C₅ to C₁₇ liquids and C₁₈ onwards solids. C₁₂H₂₆ lies in the liquid range; C₂₀H₄₂ is a solid.

Question 7
The correct order of increasing boiling point is
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Answer: (b) butane < 2,2-dimethylpropane < 2-methylbutane < pentane

Among the C₅H₁₂ isomers branching lowers the boiling point: pentane 309.1 K, 2-methylbutane 300.9 K, 2,2-dimethylpropane 282.5 K. Butane (molar mass 58 u) boils at 272.4 K, below even the most branched pentane, because the larger molecular mass outweighs the effect of branching here.

Question 8
In the photochemical chlorination of 2-methylpropane, (CH₃)₃CH, the hydrogen atom that is replaced at the fastest rate is
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Answer: (c) the hydrogen on the tertiary carbon

The rate of replacement of hydrogens in alkanes is 3° > 2° > 1°. The single hydrogen on carbon 2 is on a tertiary carbon; the nine methyl hydrogens are primary.

Question 9
Which of the following is a chain-propagation step in the chlorination of methane?
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Answer: (a) ĊH₃ + Cl–Cl → CH₃–Cl + Ċl

A propagation step consumes one free radical and produces another, so the chain continues: the methyl radical takes a chlorine atom and releases Ċl. Homolysis of Cl₂ is initiation; the two radical–radical combinations remove radicals and are termination steps.

Question 10
One mole of a hydrocarbon needs 6 mol of dioxygen for complete combustion and gives 4 mol of carbon dioxide. The hydrocarbon is
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Answer: (c) C₄H₈

For CₓHy, moles of CO₂ = x = 4 and moles of O₂ = x + y/4 = 6, so y/4 = 2 and y = 8: C₄H₈. C₄H₁₀ needs 6.5 mol O₂, C₄H₆ needs 5.5 mol, and C₃H₈ gives 3 mol CO₂.

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